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Vibro-acoustic testing on the high seas
CETENA Ship Research Center uses LMS Test.Lab for a variety of standard – and not so standard maritime equipment testing. Everything from worst-case scenario shock tests to the acoustic resonant frequency of a Murano chandelier on a mega-cruise ship is subject to the expert scrutiny of the CETENA testing team in Genoa, Italy. And this team counts on the powerful LMS testing solutions that can deliver results in minutes instead of days as billion-euro projects hang in the balance.  Like the ocean itself, testing sea-going vessels for vibration and acoustics is an awesome world all its own. Measurement systems and analysis software are much the same as those used in automotive, aerospace and other industries. Obviously, the scale and scope of testing ships is far beyond that of most other applications, since these immense and complex machines are the world’s largest moving structures.
Room-size on-board equipment such as generators and pumps are likewise massive – and noisy – with structureborne vibrations traveling throughout the ship’s all-metal skeleton and skin unless muffled at the source. Also, water-borne noise can travel for miles, potentially disturbing aquatic life and posing a security threat for naval vessels. Another safety concern is that equipment must continue operating without failure, even under extremely adverse conditions.
Qualification tests to ensure compliance with these tough requirements must be completed quickly and accurately to the strictest of standards – with naval officers, ship designers and equipment suppliers waiting anxiously for information. They want results fast and need answers now. Any inconsistencies, mistakes or missed deadline can potentially delay any one of countless milestones for a maritime project worth billions of euros.
Tools for high-stakes testing In this business, pressures are unrelenting, stakes are high and there is little room for error. But it’s all in a day’s work for engineers at CETENA S.p.A. who count on LMS Test.Lab solutions for qualification and vibro-acoustic tests. The research center is a subsidiary of Italy-based Fincantieri S.p.A., one of the world’s largest shipbuilding groups and world leader in the cruise ship market. The firm designs and builds cruise ships, ferries, merchant ships and a wide range of naval vessels. The LMS test system at CETENA consists of a 24-channel LMS SCADAS III and LMS 24-channel SCADAS mobile dataacquisition system with LMS Test.Lab software for signal conditioning, data analysis, results display, test control and report generation. With this LMS Test.Lab environment, CETENA performs qualification tests on shipboard equipment such as valves, electrical generators and electronic control systems in its state-of-the-art lab located at Fincantieri’s Riva Trigoso Shipyard in Genoa, Italy. The tests validate that components will maintain normal operation while subjected to shipboard shock and vibration.
Studying worst-case scenariosShock tests are performed to study the effects of an underwater explosion, a collision or other one-time event involving extreme transient force and displacement loads. A series of six shocks are applied by releasing a 1,500-kg hammer that impacts a heavy steel platform on which the equipment specimen is mounted. When the hammer is released, a loud “thud” reverberates through the lab as the platform and equipment are jolted. To examine these shocks, CETENA uses the LMS Test.Lab Shock Response Spectrum module, which accounts for input acceleration over a time step to calculate the shock response spectrum using measured signal data acquired from accelerometers mounted at locations across the test specimen. The analysis consists of two steps: calculation of an impulse correction to account for variations in input loads, followed by primary and residual response calculations in terms of acceleration, velocity and displacement.
Integrating signal analysis calculations within the data acquisition system increases test productivity and avoids the limitations of data conversions and interoperability difficulties encountered with multiple systems. Also, the ease with which tests can be set up and executed allows CETENA’s engineers to more readily delegate routine tests to a staff of technicians. Interactive worksheets and templates for the various types of qualification tests guide personnel step-by-step through the test process with interactive prompts and numerous default settings. A graphical user interface shows prompts that indicate what’s to be done, so for many applications technicians point and click to configure tests. In this way, the group manager can more fully utilize the different skill sets of the testing staff, with technicians running fairly routine tests very quickly and engineers focusing on solving more complex problems and coordinating projects.
Dual roles: vibration control and measurement
For vibration qualification, LMS Test.Lab serves a dual purpose in controlling the frequency and amplitude of the CETENA electro-hydraulic vibration table as well as acquiring response signals from accelerometers mounted on the test specimen. The table can test a maximum load of 1,100 kg through a frequency range of 2 to 100 Hz and stroke amplitude of 10 mm.
First, engineers do a vibration sweep to find the resonant frequency at which structural vibration response is maximum. Next, this frequency is applied to the table for a minimum of two hours to ensure that the equipment under test remains running and fully functional – without breaking or malfunctioning. “We can’t trust these critical tests with any ordinary system,” said Senior Test Engineer Stefano Qualich of the CETENA Sea Trials Department. “We need the precise control and data acquisition speed of the LMS solution to accurately control the test profiles so they deliver precisely the right frequency and amplitude profiles. Previously, this was all controlled manually; our technicians spent their time continuously adjusting vibration inputs. Doing this automatically is much more efficient and less error-prone. Also, the system quickly spots potential test problems to avoid damaging expensive hardware.” Identifying noise sources with acoustic holography
One of the most difficult challenges in marine testing is measuring underwater sound pressure levels and identifying which pieces of on-board equipment are responsible for these hydro-acoustic vibrations. Ordinarily, noise source identification on a ship is a tedious, time-consuming process as engineers contend with interference between the sounds, a multitude of resonances throughout the structure and other variables associated with the multiple frequencies and coupled vibrations. Traditionally, marine engineers often can only make rough approximations of where sounds originate and how much of a contribution they make to the overall underwater noise.
CETENA overcomes these drawbacks by using the LMS Stationary Acoustic Holography solution to automatically perform back propagation processing on measured sound pressure data – a technique using Principal Component Analysis to decompose the holographic measurements into a set of mutually independent points sources representing the equipment. Hydro-acoustic calculations are based on a combination of sophisticated symmetrization, filtering and routines to provide detailed source identification – even for multiple noise sources and wide ranges of frequencies. With equipment running on the ship, underwater measurements are taken with a set of hydrophones. Measurements are taken in patches using a suitable sensor array submerged by crane into the water and positioned 1.5 m from the hull. Starting from the bow and moving longitudinally toward the stern, measurements are taken every 0.5 m. At the same time, vibration measurements are made with a set of eight accelerometers positioned on the ship’s equipment. LMS Test.Lab then correlates and pieces together these patches to create a combined 3-D hydro-acoustic radiation map of the ship hull together with an identification of the location, frequency and vibration amplitude of each noise source. The entire test is carried out using the 24-channel LMS SCADAS Mobile unit with its integrated LMS Test.Lab suite of tools for measurement, signal conditioning, results analysis and data management – all in a lightweight laptop size unit with built-in battery power.
“Of particular value with the LMS Stationary Acoustic Holography solution is its ability to simultaneously capture and analyze phased data – that is, different effects of vibration events occurring at the same time. The LMS system manages the results in an integrated manner and displays phased data together on the same screen so engineers can see what’s happening at the same time on different channels and track down the root cause of the issue. This would be impossible with a separate non-integrated system,” explained Stefano Qualich.
Results in minutes, not days
One of the major advantages of the advanced LMS test solution is that test results are displayed very quickly after the test is run. “Parallel processing enables the system to acquire and analyze test data in near-real time so that results are available in minutes right after the test is run instead of waiting days with our previous recording system while recorded data was carried back to the lab for post-processing and analysis,” added Stefano Qualich. This on-the-spot feedback allows engineers to provide preliminary results almost immediately to anxiously waiting customers, meeting critical project milestones without delay – something that CETENA’s customers place a high value on. Moreover, engineers know quickly if the test was valid or not, thus preventing test set-ups to be dismantled prematurely and having to start the process again from scratch. “LMS Test.Lab certainly results into greater productivity,” explained Roberto Porcari, Manager of the methodologies and Trials technical area. “More broadly, this fast-turnaround testing radically improves our response times – providing results and analysis in hours instead of days. This way we work much more closely with our customers and this strengthens our leading position in the maritime industry and the trust customers place in CETENA as a world leader in maritime testing.”
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